Biohydrogenation and digestion of long chain fatty acids in steers fed on different sources of lipid

Rumen biohydrogenation of dietary polyunsaturated fatty acids (PUFA) is a s ignificant limitation on any attempt to manipulate the PUFA content of rumi nant products (meat or milk). This study examined rumen biohydrogenation of PUFA, the effects of PUFA on other aspects of rumen metabolism and fatty a cid flow to and digestion in the small intestine of steers fed on different sources of lipid. Animals were fed ad libitum on grass silage and one of f our concentrates (60:40 forage:concentrate on a dry matter basis) containin g differing sources of lipid: megalac (rich in C16:0; M), linseed (rich in C18:3n-3; L), fish oil (rich in C20:5n-3 and C22:6n-3; FO) and a mixture of linseed/fish oil (LFO). Diets were formulated so that total dietary oil in take was approximately 60 g/kg of the DM intake, approximately half of whic h was from the experimental test oil. Rumen NH3-N (P = 0.09) and total VFA concentrations (P = 0.007) were higher on L. FO and LFO compared to M. Dry matter intakes did not differ across treatments and averaged 7.2 kg/day. In take and flow of fatty acids to the duodenum was 323, 438, 344 and 381 (S.E .M. 9 1; P < 0.001) and 432, 489, 412 and 465 (S.E.M. 18.5; P < 0.1)g/day f or M, L, FO and LFO, respectively. Biohydrogenation of C18: 1n-9 was lower than all the other unsaturated fatty acids and it was lower of FO and LFO c ompared to M and L, on average 66.1 and 72.2%, respectively. Biohydrogenati on of C18:2n-6 averaged 89.8% across treatments and was lower (P < 0.05) on M compared to L and FO. Biohydrogenation of C18:3n-3 averaged 92.1% across treatments and was lowest on M (88.8 %) and highest on L and LFO (94.3%). Biohydrogenation of C20:5n-3 and C22:6n-3 averaged approximately 91 and 89 % across the treatments, respectively. Small intestinal digestibilities of all fatty acids were high. In conclusion, feeding different sources of lipi d with different fatty acid composition had significant effects on rumen fu nction. The PUFA in whole linseed were only partially protected from biohyd rogenation by the seed coat and in contrast to previous reports the C,, PUF A in fish oil were biohydrogenated to a large extent.